1. Field of the Invention
The present invention relates to a magnetic storage device equipped with a function of assisting magnetization reversal of a perpendicular magnetic recording medium by applying high-frequency magnetic field, and a method for controlling the same.
2. Background Art
The growth of the Internet environment and newly provided data centers along with penetration of cloud computing have increased the amount of information generated in recent years. There is no doubt that magnetic storage devices such as magnetic disk devices (HDD) having the highest recording density and excellent bit cost play the leading role for storage in the “age of big data”. To this end, magnetic storage devices have to have larger capacity and further increasing recording density supporting this is must.
As a magnetic recording technique for higher-density recording, microwave assisted magnetic recording (MAMR) has been proposed, in which high-frequency magnetic field in a microwave band is applied to a perpendicular magnetic recording medium so as to excite precession of medium magnetization for magnetic recording on a perpendicular magnetic recording medium having large magnetic anisotropy while reducing the switching magnetic field. In recent years, practical and micro-structured high-frequency Spin Torque Oscillator (STO) has been proposed by U.S. Pat. No. 7,616,412 B2, for example, which is configured to rapidly rotate spins by spin torque, thus generating high-frequency magnetic field. WO 2010/053187 A1 then discloses a method of making a high-frequency magnetic field oscillator generate high-frequency magnetic field (circularly polarized high-frequency magnetic field) rotating in the direction of the precession of the magnetization of the perpendicular magnetic recording medium to be magnetization-reversed so as to be suitable for the polarity of the magnetic recording field, thus effectively inducing the magnetization reversal. In this way, research and development for implementation of the microwave assisted magnetic recording has become active.
Thus the pace of research and development for implementation of the microwave assisted magnetic recording has been hastened in recent years, and JP 2011-113621 A discloses, in order to achieve the reliability of a high-frequency oscillator necessary to implement such microwave assisted magnetic recording and to keep such oscillation with reliability, a magnetic head driving control apparatus configured to, in a state of applying magnetic recording field, supply a high-frequency oscillator driving signal at a level higher than an ordinary level only for a prescribed effective time in response to an input write gate.
As techniques for perpendicular magnetic recording media, JP 2010-231866 A and JP 2006-73176 A, for example, propose a method for manufacturing a perpendicular magnetic recording medium, capable of yielding favorable magnetic properties and recording/reproducing properties, in which the direction of exhausting material gas used for sputtering film formation and the position of erosion (eroded area to be sputtered) of a target material are devised so as to improve the in-plane distribution of the magnetic properties without degrading productivity or increasing cost.
As techniques for magnetic recording devices, JP 2008-47277 A discloses a data recording method including the steps of: measuring coercive force distribution of a magnetic recording medium, and then when the ambient temperature is higher than a room temperature, preferentially selecting a recording area having a relatively high coercive force for data recording and when the ambient temperature is lower than a room temperature, preferentially selecting a recording area having a relatively low coercive force for data recording, whereby reliability of data recording can be maintained without changing the magnetic recording field of the magnetic head and irrespective of a change in temperature.